A farmer was forced to dump more than 1,200 organic chickens after they were illegally fed on genetically modified soya beans, The Independent on Sunday can reveal.

It is the first time that organic food has been stopped from going on sale in Britain because of GM contamination - alarming environmentalists and food safety groups.

The affected soya beans came from Italy, in breach of a European Union ban on the sale of any GM crops to farmers and consumers.

Food safety groups fear that other GM-contaminated feed could have gone undetected before the affected batch was discovered during routine testing at an animal feed plant in north Devon late last year.

The discovery forced the Soil Association, the UK's main organic certification body, to issue an urgent alert to affected farmers and trace all the organic farms that had bought the contaminated feed.

They found that two flocks of free-range organic chickens had been fed the GM soya, although no birds had been sold. The affected farm was stripped of its licence to sell organic chickens, and had to sell off the flocks as non-organic at a rock-bottom price.

The scare is likely to set back government attempts to persuade a sceptical British public to accept GM foods. That campaign started in earnest earlier this month when an official "national debate" on GM foods began.

The Environment Secretary, Margaret Beckett, and the Food Standards Agency are fiercely resisting proposals for a stricter limit on the amount of accidental GM contamination in foods that have a "GM-free" label. Euro-MPs want all GM-free foods to have less than 0.5 per cent GM produce in them.